In people's life and some industrial production, will involve the problem of liquid level control, but the water tank level is often automatic control effect is not good. This paper first introduces the status quo of water tank level control and control deficiencies, and then carries on the design of active disturbance rejection controller, based on the traditional PID combined with active disturbance rejection control technology. Secondly, the hardware system of this design system is designed and selected. Finally, through the lower computer programming, upper computer state, program running and upper computer screen monitoring and experimental results analysis, the control system has achieved the automatic control effect of water tank liquid level has been greatly improved.